Members of this HMM comprise a subfamily of the Outer Membrane Factor (TCDB 1.B.17) porins. OMF proteins operate in conjunction with a primary transporter of the RND, MFS, ABC, or PET systems, and a MFP (membrane fusion protein) to tranport substrates across membranes. The complex thus formed allows transport (export) of various solutes (heavy metal cations; drugs, oligosaccharides, proteins, etc.) across the two envelopes of the Gram-negative bacterial cell envelope in a single energy-coupled step. Current data suggest that the OMF (and not the MFP) is largely responsible for the formation of both the trans-outer membrane and trans-periplasmic channels. The roles played by the MFP have yet to be determined.